1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to correcting resonant optical components that are used to communicate optical signals.
2. Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, large density, low latency and low power consumption for inter-chip and intra-chip connections. In the last few years, significant progress has been made in developing low-cost components for use in inter-chip and intra-chip silicon-photonic connections, including: high-bandwidth efficient silicon modulators, low-loss optical waveguides, wavelength-division-multiplexing (WDM) components, and high-speed CMOS optical-waveguide photo-detectors. Interference-based silicon devices, such as silicon ring resonators, arrayed waveguide gratings (AWGs), and Mach-Zehnder interferometers (MZIs), have become an integral part of a silicon-photonic interconnect design.
However, interference-based silicon devices usually require precise control of the phase relationship to achieve a desired spectral response. The phase of the optical mode in these interference-based silicon devices is directly proportional to the effective index of refraction of the optical waveguide. As a consequence, small changes in the optical waveguide geometry or the index of refraction of the materials used in the core and cladding of the optical waveguide can strongly influence the resonance wavelength of an interference-based silicon device. In case of the silicon ring resonator, for example, we have the following relationship:
                    Δλ        res                    λ        res              ≈                  Δ        ⁢                                  ⁢                  n          eff                            n        g              ,where ng is the group index of refraction of the optical waveguide, neff is the effective index of refraction, and λres is the originally designed or target resonance wavelength. Therefore, a change of the effective index of refraction on the order of approximately 0.1% can induce a resonant-wavelength shift of around 1.55 nm (i.e., 200 GHz), which is larger than 3-dB resonance width of most ring resonators with Q-value higher than 1000. For a practical high-performance application, a change in the effective index of refraction of this magnitude typically cannot be tolerated. This often puts very tight constraints on the precision and purity of the fabrication process and materials used, even for the most sophisticated foundry processes currently available.
A variety of factors can change the effective index of refraction, including: variation in the silicon-on-insulator (SOI) thickness, variation in photolithography, variation in etching and/or uncertainties in the index of refraction of the materials used. For example, the variation in the thickness of standard SOI wafers, can offset the height of the fabricated optical waveguide by ±2 nm). Moreover, variations in focus, exposure energy, developing condition and/or unintentional hard bake-induced reflow of developed photoresist pattern during photolithography can change the width of the fabricated optical waveguide by ±5 nm. Furthermore, it is often difficult to control the etch depth better than ±2 nm with the typical etch rates of about 4 nm/second for most reactive-ion-etch (RIE) systems. In particular, the heating pattern and electric-field distribution across the wafer can cause variations in the etch depth, even when reduced by helium backside cooling and other methods that are intended to improve uniformity. Similarly, imperfect verticality of etched optical waveguide sidewalls can change the effective index of refraction of the fabricated optical waveguide. Additionally, the effective index of refraction can be offset by: small variations in the doping concentration of silicon material, variations in the density and stress of tetraethyl orthosilicate-based silicon oxide cladding and/or other causes. Consequently, fabrication of silicon ring resonators with precisely specified resonance wavelengths is often hard to achieve even with state-of-the-art foundry processes.
One approach for addressing this problem is post-fabrication compensation of phase errors to shift the resonance wavelength to the desired or target value (which is henceforth referred to as ‘trimming’). For example, active elements (such as micro-heaters) may be used to induce a shift in the resonance wavelength based on the thermo-optic effect. However, because of the large number of integrated devices per die and the possibility of thermal crosstalk, trimming using active elements often results in excessive power consumption.
In principle, a desired shift in the resonance wavelength can be obtained using per-device (low volume) and permanent single-shot post-fabrication trimming. However, these fabrication techniques usually are time-consuming, often have insufficient trimming range and/or the resulting trimmed resonance wavelengths typically do not have sufficient long-term stability.
Hence, what is needed is a ring-resonator modulator and an associated fabrication technique without the above-described problems.